In the operation of refrigeration and air conditioning systems, a cooling effect is provided by the change of state of the refrigerant from a liquid state to a gaseous state in the evaporator. The gaseous refrigerant is compressed by a compressor and is condensed to a liquid state in a condenser before passing through an expansion valve and being delivered to the evaporator again.
The compressor is typically an expensive piece of equipment, especially in the case of a large compressor such as one used in air conditioning systems for large commercial, industrial or office buildings. One of the principal causes of compressor damage is a condition known as "floodback" or "washout" caused by the presence of undue amounts of liquid refrigerant at the compressor intake. The liquid refrigerant cannot be compressed, and its presence in the compressor causes liquid slugging and damage to various components, including valves and pistons. The liquid refrigerant also dilutes the lubricating oil in the compressor and washes lubricant away from the bearings, cylinder walls and other surfaces that are subjected to high frictional forces. Dilution of the oil and the increase in friction shortens the life of the compressor. Compressor break down has serious consequences in that it requires costly repair or replacement of the compressor, causes inconvenience and discomfort due to lack of cooling while the compressor is out of service, and can destroy expensive material in cases where cooling is needed for a critical industrial or medical process.
The presence of unduly wet refrigerant at the compressor intake is caused by a lack of superheat on the suction side of the compressor. Conversely, if the superheat is excessive, the cooling effect of the refrigerant on the compressor is reduced. This can result in overheating of the compressor motor and/or the valves and high friction areas of the compressor. If the suction pressure is unduly low, the refrigerant gas is not present in the system in sufficient amounts to adequately cool the compressor. Adverse consequences such as overheating of the motor or other parts of the compressor can result from this condition.